As the density and power of electronic components have increased, the problem of excessive heat generation has become a significant concern to industry. Heat pipes have been found to provide superior thermal transfer characteristics for cooling electronic circuits.
In the prior art, a heat pipe often comprises a closed vessel or chamber whose inner surfaces are lined with a porous capillary wick that is saturated with a working fluid. The heat pipe has an evaporator section that absorbs heat and a condenser section where the heat is released to a heat sink in contact with that section of the heat pipe. In operation, heat absorbed by the evaporator section causes liquid to evaporate from the wick. The resultant vapor is transferred within the vessel to the condenser section of the heat pipe where it condenses releasing the heat of vaporization to a heat sink. The capillary action of the wick pumps the condensed liquid back to the evaporator section for re-evaporation. The process will continue as long as working fluid is contained within the heat pipe.
Sometimes, the working fluid in the heat pipe chamber is lost due to a breach of the heat pipe's wall. Such a breach often occurs at the point where the working fluid was introduced into the heat pipe. The ability to reliably and effectively seal heat pipes has been sought by the industry for many years, because if the fluid within the heat pipe is lost, the equipment cooled by the heat pipe could be subject to significant heat damage. Several means of sealing heat pipes have evolved over the last couple of years.
In one conventional arrangement, a heat pipe includes a hollow tube with end caps inserted into each end of the vessel. One end cap has a hole therethrough with a copper pinch-off tube brazed to the hole. The heat pipe is purged and filled with the proper working fluid using the copper tube. To seal the heat pipe, the copper tube is pinched shut using a roller pinch off tool or the like. See, for example, Dunn & Reay, Heat Pipes 154 (3rd Ed. 1982). However, the rollers of the pinch off tool get close to the braze and may crack the braze during pinch off. Additionally, after being sealed the fragile copper tube protrudes outwardly a short distance from the end cap, and therefore is very susceptible to breakage. In order to adequately protect this protruding copper tube, a cover must be placed over the end cap and copper tube. The end cap cover and copper tube disadvantageously consume a large portion of the condenser section at the end of the heat pipe. Both reliability and efficiency of the heat pipe are limited by this technique.
In an attempt to improve upon this design, the copper tube has been attached directly to the side of the heat pipe vessel instead of to the end cap. In this prior art arrangement, a copper tube is welded into a hole within the side of the heat pipe vessel, and the heat pipe tube chamber is purged and filled with working fluid using this copper vessel. After filling the heat pipe with fluid, the copper tube is pinched shut to seal the vessel. As with the above-described process, the weld can be cracked during pinch off. Furthermore, this sealing technique is disadvantageous in that a portion of the copper tube extends outwardly from the side of the heat pipe. In this arrangement, the fragile copper tube has no cover and is very susceptible to breakage. Additionally, the placement of the copper pinch-off tube on the side of the heat pipe vessel hampers expulsion of non-condensable gases during purging. Furthermore, because the copper tube protrudes outwardly from the side of the heat pipe, heat pipes formed by this technique cannot be placed adjacent to each other.
Consequently, there is a need in the art for an improved heat pipe which is economically accomplished, and provides a strong and reliable seal.